Theoretical studies of hydrogen bonding in liquid water and dilute aqueous solutions

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Abstract
Monte Carlo computer simulations of liquid water and dilute aqueous solutions are analyzed in terms of the nature and extent of intermolecular hydrogen bonding. A geometric definition of the hydrogen bond is used. Calculations on liquid water at 25 °C, 37 °C, and 50 °C, were carried out based on the quantum mechanical MCY potential of Matsuoka, Clementi, and Yoshimine and at 10 °C based on the empirical ST2 potential. The effect of a dissolved solute on aqueous hydrogen bonding was studied for dilute aqueous solutions of Li+, Na+, K+, F−, Cl−, and CH4. The nature of the hydrogen bonding was characterized with quasicomponent distribution functions defined as a function of the intermolecular coordinates relevant to hydrogen bonding. The extent of the hydrogen bonding is described using a network analysis approach developed by Geiger, Stillinger, and Rahman. The results on the quasicomponent distribution functions show that the average hydrogen bond angle deviates with 10 °–25 ° from a linear form, quite independently of the potential function. The decrease in icelike character in liquid water as the temperature is increased is quantified. The structuration effects in solvent water for dilute aqueous solution of ionic and hydrophobic solutes are computed, and interpreted in terms of the Frank and Wen A, B and C regions of solvent. There is increased structure in the A region of both the ionic and apolar solutes, electrostrictive for the former and clathratelike for the latter. The B region is destructured in ionic solutions and shows increased icelike structural character in the solution of an apolar solute. The network analysis showed the existence of large space‐filling hydrogen bonded networks. The occurrence of monomers was found to be negligibly small. These findings are in quantitative agreement with the analysis of molecular dynamics results by Geiger et al., based on an energetic hydrogen bond definition. Furthermore, the parameters of the distributions of the hydrogen bonded networks were found to be remarkably invariant to small change in the temperature, introduction of a solute and the change of the potential function. The average cluster size and related parameters are quite sensitive to the number of molecules considered. Overall, the analysis supports the validity of viewing liquid water in terms of a Pople continuum and Sceats–Rice random network model of water molecules interacting mainly via bent hydrogen bonds.